There has been known a ceramic substrate having a ceramic layer mainly formed of a glass ceramic and a conductor trace containing silver (Ag) as a main component. Such a ceramic substrate is formed by applying a conductor paste which is the pre-firing form of the conductor trace to a green sheet which is the pre-firing form of the ceramic layer and then firing the green sheet carrying the conductor paste applied thereto. Such a ceramic substrate is also called a low temperature co-fired ceramic (LTCC) substrate.
When the ceramic substrate is formed by firing, a silver component of the conductor paste diffuses into the ceramic layer. This may cause, for example, formation of voids in the ceramic layer, deformation of the ceramic layer, and change of the color of the ceramic layer. It is considered that diffusion of the silver component into the ceramic layer is accelerated by oxidation of the silver component of the conductor trace.
Patent Document 1 discloses a technique of preventing diffusion of the silver component into the ceramic layer by covering the surfaces of silver powder particles contained in the conductor paste with an antimony salt. Patent Document 2 discloses a technique of preventing diffusion of the silver component into the ceramic layer by adding a silicon powder to the conductor paste.